Enzyme Mediated Encapsulation of GoldNanoparticles by Polyaniline Nanoshell

R. Sfez#1,2, E. Natan#3, Y. Bardavid1, M. Ikbal1, E. Arbeli4,Isaiah T. Arkin5, I. Popov6 and Shlomo Yitzchaik1,6∗

1Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem91904, Israel2Azrieli, College of Engineering, Jerusalem, Israel3Department of Chemistry, University of Oxford, Oxford OX1 3TA, UK4Department of Chemistry and the National Institute for Biotechnology in theNegev, Ben-Gurion University of the Negev, Be’er-Sheva 84105, Israel5Department of Biological Chemistry, The Alexander Silberman Institute of LifeSciences, The Hebrew University of Jerusalem, Edmund J. Safra Campus,Jerusalem, 91904, Israel6The HU Center for Nanoscience and Nanotechnology, The Hebrew University ofJerusalem, Jerusalem 91904, Israel#These authors contributed equally to this work*Corresponding author: sy@cc.huji.ac.il

Received 14 January 2015; Accepted April 7 2015;Publication April 14 2015

Abstract

In this contribution we describe the formation of gold nanoparticles (AuNP)and polyaniline (PANI) AuNP-PANI nanocomposite via in situ enzymaticpolymerization. The method consists of electrostatic adsorption of aniliniummonomers on AuNPs citrate stabilized surface of 50 nm diameters, followedby oxidation with horseradish peroxidase (HRP) enzyme and its cofactorH2O2.All reaction steps were monitored by UV-Vis-NIR spectroscopy includ-ing in situ detection of the polymerization process. UV-Vis-NIR, Cyclicvoltammetry (CV) and surface enhanced Raman scattering (SERS) measure-ments supported the formation of a nanoshell of PANI on the AuNP core.

Journal of Self-Assembly and Molecular Electronics, Vol. 3, 1–16.doi: 10.13052/jsame2245-4551.311c© 2015 River Publishers. All rights reserved.

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Two templates for anilinium assembly were compared revealing a strongdependence of the enzymatic kinetics on the template. The kinetic study hadshown that the rigid template of the AuNP contributes to higher reaction rateon the AuNP compared with the more flexible polyanion template. The mildreaction condition enables an easy and precise method for obtaining PANInano-shell on anionic templates for advanced bioelectronic applications.

Keywords: Au-nanoparticles (AuNP), enzymatic polymerization, HRPenzyme, polyaniline, nano-shell.

1 Introduction

Gold nanoparticles (AuNP) among other noble metal nanoparticles haveattracted considerable attention due to possible applications in numerousfields, such as catalysis, electronics, optics [1] and much more [2]. Conjugatedpolymers, also known as intrinsically conductive polymers, are widely used invarious applications due to their ease of synthesis, low cost and facile controlof their optical and electronic properties [3]. Therefore, it is to be expectedthat hybrid materials composed of both metal NPs and conducting polymerswould have a wide range of applications and interesting properties. Indeed,those hybrids which were recently reviewed [4] are already used among otherapplications for biological [5], electronic [6], catalysis [7] and devices [8].Polyaniline (PANI) is one of the most used conducting polymers since itsdiscovery a few decades ago because its conductivity depend on both itsredox state and proton doping, i.e. the pH of the environment [9]. The useof anionic templates for adsorption of positively charged monomers followedby in situ chemical, electrochemical and enzymatic polymerization, yieldingsurface confined nanolayers of conducting polymers was already exemplifiedfor various substrates, such as: metal [10] and oxide surfaces [11], poroussilicon [12] carbon nanotubes [13] and DNA [14]. These experiments aresetting the stage for the use of new in situ template synthesis on negativelycharged AuNPs.

In this work we have chosen to focus on a core shell hybrids composedof a 50 nm AuNP core, covered by a monolayer shell of in situ enzymaticallypolymerized PANI. This specific hybrid might be of interest in variousapplications, like in memristive devices [15a] and as a good template forneuron growth [15b]. The synthetic method consists of electrostatic ads-orption of positively charged anilinium monomers on the negatively chargedNP surface followed, by oxidation with horseradish peroxidase (HRP)

Enzyme Mediated Encapsulation of Gold Nanoparticles 3

enzyme and its cofactor H2O2. Typically, in order to build nanocompositesof Au core and PANI shell many methods have been used, among which,the use of conducting polymers as reductants [4] or the use of the aniliniummonomer as a reductant of the metal salt while oxidative polymerization takesplace [16]. Of course, electrochemical methods are widely used also [17]. Ingeneral, it is very difficult to obtain a controlled monolayer nanoshell on NPmainly due to aggregation problems during the coating process or growthpattern which is dictated by the organic component. Some attempts wereconducted on AuNP-PEDOT [18] or Au-polypyrrole hybrids [19], and alsowith polyaniline using chemical oxidation [20]. In our work, we wanted tobuild a controlled nano-shell of PANI on Au-NP core by in situ enzymaticpolymerization which afford moderate pH environment (pH 4.3), and controlover the reaction conditions and product by governing the thickness of theformed PANI monolayer. The motivation for this research arises from the needto clarify fundamental aspects of enzymatic polymerization at nanometricconfined geometries and characterization of the obtained nano-composite.Enzymatic polymerization of anilinium monomers electrostatically attachedto poly (4-styrenesulfonate) (PSS) using HRP was described few years ago[21] and extensively used since then. The use of enzymes as catalysts inchemistry is wide spread [22] due to their specificity and their ability toafford reactions’ control by using inhibitors, temperature and pH modulationwhich alter their reactivity dramatically. HRP is comprehensively used forredox reactions including anilinium polymerization, allowing a relatively highpH environment (up to 4.3) compare the typical pH environment which ismuch lower (1–2 for electrochemical and chemical oxidations). Followingprevious works [23] which showed the activity of HRP enzyme on varioustemplates including 2D modified surfaces [24], our goal being to try andelucidate the role of the template on the action and kinetics of the enzymaticreaction while obtaining the desired nanoshell of PANI around Au-NP core.Two templates were compared for polymerizing anilinium monomers: thepolyanion PSS and 50 nm Au-NP. These templates differ dramatically intheir rigidity. Three experiments were conducted in which systems withoutany template, with 50 nm Au-NP template and with PSS linear polyaniontemplate were compared. In the template containing experiments, aniliniumwas electrostatically adsorbed to the template surface (by sulfonate groupsin the case of PSS or by carboxylate groups in the case of the citrate capped50 nm Au-NP). Enzymatic polymerization took place after proper dialysisprocess which eliminates polymerization of unabsorbed monomers. It hadalready been proved that the most important role of the polymeric template

4 R. Sfez et al.

in enzymatic polymerization is to provide the preferential orientation of theanilinium in order to get the head to tail polymerization [25]. We have find bycomparing the different templates that the rigidity parameter is important fromkinetics point of view, as could be expected. Indeed it was already recentlyreviewed [26], that assembling enzymes or substrates to metal nanoparticleshas an important influence on the kinetics of the system. All reaction stepswere monitored by UV-Vis-NIR spectroscopy including in situ detection ofthe polymerization process. HR-TEM analysis gave morphological analysisof the monomeric and polymeric layer on the Au-NP. Surface enhancedRaman scattering (SERS) analysis was conducted in order to compare themonomeric form and polymeric form of the nanolayer coating the NP. Cyclicvoltammetry (CV) measurements were conducted on PANI coated AuNPattached to modified ITO surfaces, acting as a working electrode, and gaveevidence for the presence of the PANI layer. The kinetic role comparison ofthe different templates was conducted by UV-Vis-NIR spectroscopy usingtime course mode. The kinetic study had shown that the rigid template of theAuNP contributes to higher reaction rate on the Au-NP compared with themore flexible polymeric template with the same surface number density ofanionic (sulfonate) groups.

2 Experimental Details

2.1 Chemicals

HAuCl4, PSS (MW = 70,000) HRP (Aldrich), were used as received. Aniline(Aldrich) was vacuum distilled before use.

2.2 Synthesis

Gold nanoparticles capped with citrate ligands were synthesized as wasdescribed elsewhere [27]. Briefly, a solution of 0.01% of HAuCl4 was mixedand heated with changing volumes of 1% wt. of sodium citrate solution,resulting in the formation of AuNP capped by citrate ligands.

The in situ enzymatic polymerization process took place on the aniliniumcovered Au-NP. In order to obtain this composite, anilinium solution (50 mM,pH 4.0) was added drop wise to synthetized citrate covered Au-NP followedby a 30 min incubation in an ice bath (4oC) (Scheme 1, step 1). Followingprevious works which describes aggregation of anilinium [28] a thoroughdialysis process was included. Dialysis was carried out against a volume of

Enzyme Mediated Encapsulation of Gold Nanoparticles 5

Scheme 1 Formation mechanism of Au-NP-PANI nanocomposite via enzymatic polym-erization. Step 1: electrostatic adsorption of positively charged anilinium monomers tonegatively charged NP. Step 2: enzymatic polymerization of anilinium nanoshell convertedto PANI nanoshell

1 litter HCl (pH 4.0) solution, three times for 15 minutes, in order to eliminateunadsorbed anilinium monomers and avoid their bulk polymerization.

The anilinium covered Au-NP was then let to react with HRP (1.2 μg/ml,room temprature) and H2O2 (1.2 nM, added drop wise) solutions (scheme 1,step 2) for different periods of time, ranging from 10–90 min. The productwas characterized by HR-TEM, UV-Vis and SERS analysis. All reactionswere monitored by in situ UV-Vis-NIR spectroscopy. While using PSS as atemplate for kinetics study a known procedure was used [21] with dialysis asmentioned above.

PSS modified ITO substrates for CV analysis were prepared according toa procedure described elsewhere [24]. Briefly, ITO electrodes were modifiedwith APTMS by using 1% solution of APTMS in methanol followed bywashing with methanol and drying with N2. APTMS modified ITO substratewas placed in aqueous solution of PSS for 1 hour and rinsed with water. Thenegatively charged PSS modified substrate was incubated in PANI coatedAu-NP solution overnight allowing electrostatic adhesion of the positivelycharged core-shell NPs.

2.3 Instrumentation

UV-Vis-NIR spectra were acquired with a Shimadzu spectrophotometer UV-3101PC. HR-TEM measurements were conducted with a Tecnai F20 G2

operating at 200 KeV including Energy Dispersion X-ray Spectroscopy(EDS) analysis of elements. A 400 mesh size carbon coated copper gridwas used for the analysis. Samples for SERS were excited in a back-scattering geometry with 488 nm irradiation from an INNOVA 90CFRED Argon-Ion laser (Coherent Inc., Santa Clara, CA, USA). Spectra were

6 R. Sfez et al.

recorded by a T64000 triple Raman spectrometer (Jobin-Yvon Horiba,Longjumeau, France) equipped with a liquid nitrogen-cooled Spectrum OneCCD detection system (Jobin-Yvon Horiba) in the range of 400 cm−1 to1700 cm−1 with the spectral resolution of 3 cm−1. Raman frequencies werecalibrated using the 934 cm−1 band of NaClO4. The reported spectra are theaccumulated averages of 25 exposures of 3 sec each. For all measurements,the spectra of Au nanoparticles in acid solution (HCl, pH 4.0) was takenas a background. Electrochemical measurements were carried out by usingAUTOLAB PGSTAT12 (Eco Chemie B.V). Modified ITO substrate was usedas a working electrode, a platinum wire as a counter electrode, Ag/AgClsaturated KCl as a reference electrode, and 0.1 M HCl was used as supportingelectrolyte.

The cyclic voltammograms (CV) were recorded by applying voltagebetween 1.0–0.0 V at 50 mV/s scan rate.

3 Results and Discussions

HR-TEM analysis was conducted on both the anilinium monomeric andPANI polymeric coated NP after a proper dialysis process. The first stepconsisted with the formation of the monomeric layer (Figure 1a) of aniliniumelectrostatically bound to the negatively charged NP. The obtained layer isimaged as a thin nanoshell that can be estimated to be of about 2 nm widthafter drying. TEM measurements indicated that no aggregation have occurred.Moreover, the characteristic peak of N 1s was observed only on the hybridbut not on the grid as indicated by EDX (Suppl. 1). These results support theclaim that no effective concentration of anilinium remained in the solution afterthe consecutive dialysis. The second step consisted of the in situ enzymaticoxidation and polymer formation. The in situ enzymatic polymerization wasconducted on the anilinium free solution containing anilinium covered NPat pH 4.0. Different time periods were examined and fifteen minutes wasdetermined to be the optimal time in order to get polymerization and avoidaggregation.

A fifteen minutes polymerization time resulted in the formation of a thinlayer of the polymer (Figure 1b).Almost no change in the nanoshell width wasobserved upon increasing polymerization time, supporting our assumption thatno free anilinium monomers remained in the solution after dialysis. However,upon polymerization more hybrids were observed as pairs or small aggregates.This fact can be explained by the polymerization which takes place on oneparticle and continues on the nearest particle where anilinium monomers could

Enzyme Mediated Encapsulation of Gold Nanoparticles 7

Figure 1 HR-TEM images of (a) 50 nm Au-NP coated by anilinium monomer nanoshell.(b) PANI nanoshell after 15 min of in situ enzymatic polymerization. (c) same as (b) for agroup of NPs after polymerization (Scale bar 20 nm)

be still polymerized (Figure 1c). While letting a longer reaction time we gotdifferent polymeric matrices embedded Au-NP aligning the NP was observed.It is important to notice that no trace of the enzyme was observed in the finalproduct. While replacing the HRP enzyme by a mutant HRP lacking of activesite no polymerization was detected, as was already proved for surfaces [24].

UV-Vis-NIR analysis was conducted on the monomer covered NP givingrise to the Au surface plasmon at 537 nm as expected by the size of theNP (Figure 2), indicating that no aggregation had occurred (see Supple. 2,for detailed UV-Vis). The enzymatically oxidized anilinium-monomer coatedNP gave rise to a broad spectra indicating PANI formation (Figure 2), and

Figure 2 Comparison of UV-Vis.-NIR spectra of PANI polymer obtained on the differenttemplates: (a) anilinium coated Au-NP showing characteristic plasmon at 537 nm, (b) PANIcoated Au-NP (30 min), (c) anilinium coated PSS, and (d) PANI coated PSS (30 min)

8 R. Sfez et al.

red shift of the surface plasmon, probably due to polymer formation. Whilecharacterizing the PSS template for polymerization, UV-Vis-NIR analysishas shown clearly that the obtained polymer on both templates is the same(Figure 2).

The only difference between the spectra is the Au surface plasmon whichappears in the Au-PANI nanocomposite, but lack on the PSS, as expected.SERS analysis was conducted on samples with higher concentration thanwas needed for TEM and UV-Vis. While comparing the SERS analysisobtained for the monomer and the polymer, two main features were observed(Figure 3). The first one is the broadening of the peaks for the polymercomparing the monomer (sharp peak at 1200 cm−1 for the monomer com-paring broad peak from 1100 cm−1 to 1350 cm−1 for the polymer) suggestingoligomer or polymer formation. The second feature is the newly appearingpeaks characteristic of quinoid group (such as C-C bond in quinoid ring at1580 cm−1 or C-H bending in quinoid ring at 1158 cm−1) which exist in thepolymer but lack in the monomer, thus supporting polymer formation [29].

CV measurements were conducted on the modified ITO electrodeimmersed in HCl solution (pH = 4).As can be shown (Figure 4) a redox coupleappears at 0.30 V (cathodic) and 0.48 V (anodic) which was attributed to the

Figure 3 Raman spectra of (a) the anilinium monomer and (b) Polymer adsorbed over theAuNP’s, at λ = 488 nm. The presented spectra are obtained following background subtraction(AuNP’s solution)

Enzyme Mediated Encapsulation of Gold Nanoparticles 9

Figure 4 CV of Au-PANI nanocomposites electrostatically adsorbed to PSS modified ITOsurface. Modified ITO surface was used as a working electrode, a platinum wire as a counterelectrode, Ag/AgCl saturated KCl as a reference electrode, and 0.1 M HCl was used assupporting electrolyte. Scan rate for the range of 1.0–0.0 V at 50 mV/s

reduction and oxidation process of the PANI nanoshell, although a bit shiftedcomparing PANI characteristic values [10, 11a]. These results are similar toPANI polymerized on nano-confined templates [30], and support the forma-tion of PANI nanoshell on Au-NP. Control experiments were conducted withnon-coated Au-NP that gave no PANI signal mainly due to the electrostaticrepulsion between the PSS modified ITO and the negatively charged particles.The PANI nanolayer obtained therefore is not stable and thus explaining thelack of redox peaks in the voltammogram.

Kinetics study was carried out with time course spectroscopic analysiswhile choosing the wavelength to be 709 nm. This wavelength was chosenbecause it is characteristic of PANI’s bipolaron absorption, which increaseswith time.

As can be seen (Figure 5), the control experiment which was conductedwithout a template shows no significant change with time. However, compar-ison of the two templates, i.e. Au-NP and PSS, show an increase in the peakintensity but with different growth rates.

The rate of PANI growth on the rigid NP is higher than thegrowth on the more flexible linear PSS by a factor of two

(kAu

obs. =3 .95 · 10−5 sec−1 vs. kPSS

obs. = 2.06 · 10−5 sec−1). This difference in rigidity

can account for the different enzymatic rate of polymerization. The mechanism

10 R. Sfez et al.

Figure 5 UV-vis. driven kinetic experiment tracing 709 nm with time for different exper-imental conditions of enzymatic polymerization: (a) the AuNP’s template (b) PSS template(c) no template

of the enzyme action remains unclear; however we think that we can considerthe action like the action on a 2D surface due to the fact that the enzymeactive site is about 20Å3, and so the NP which has a diameter of 50 nm canbe considered as a “flat” surface. Comparison of chemical and enzymaticoxidation shows the advantages of the enzymatic polymerization with regardto the condition of the polymerization and the control over the reaction andthe obtained aggregation free PANI encapsulated Au-NP.

4 Summary

We had shown a way to get nano-composites of Au-NP-PANI by in situenzymatic polymerization achieved in facile and moderate conditions, usingHRP in the presence of H2O2. Structural characterization showed clearly ananolayer formation of in situ enzymatically polymerized PANI. CV, UV-Vis-NIR and SERS measurements supported the formation of a nano-shell ofPANI on the Au-NP. It is a first example of in situ enzymatic polymerization ofanilinium monomers to PANI on Au-NP. The role of the template was studiedkinetically and had shown a dependence of the enzymatic reaction rate on therigidity of the template. This moderate method can be used in the future as atool for gentle synthesis with various NP’s and many horizons in bionic andnanoelectronic driven devices.

Enzyme Mediated Encapsulation of Gold Nanoparticles 11

Acknowledgements

This work is supported by the FP7 318597 SYMONE project (FET-UCOMPthematic area) and in part by COST action MP1202 (HINT).

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Biographies

R. Sfez got her Ph.D. in Chemistry at the Hebrew University of Jerusalem, in2003. She continued as a researcher at the university for several years. Since2011 she is a senior lecturer at theAdvanced materials engineering departmentand head of academic studies in Chemistry at Azrieli, college of engineering,Jerusalem. Her current research interests are self- assembled monolayers forvarious applications and nanomaterials hybrids.

E. Natan received his BSc. in Biology, and later a MSc. in Organic Chemistryunder the supervision of Professor Shlomo Yitzchaik, both at the HebrewUniversity of Jerusalem. He conducted his PhD in Cambridge University inthe lab of Professor Alan R Fersht and joined as a postdoc to the Teichmannlab in the EMBL European Bioinformatics Institute. His research activity isin the field of protein structure, folding and stability.

14 R. Sfez et al.

Y. Bardavid was born in Istanbul Turkey,1978. He got his Ph.D. from HebrewUniversity of Jerusalem in 2010. He is working at Nanonics, an IsraeliNanotechnology company since 2010 as a researcher.

M. Ikbal was born in West Bengal, India, February 1983. In 2005 he got hisB. Sc degree in Chemistry from Vidyasagar University, India and from sameUniversity he received his M. Sc degree in Organic Chemistry in 2007. Heobtained his Ph.D. degree in Organic Photochemistry, under supervision ofProf. N. D. Pradeep Singh, from Indian Institute of Technology Kharagpur.Currently he is a Postdoctoral Fellow in the group of Prof. Shlomo Yitzchaik.His recent research focuses on the design and fabrication of memristiveelements based on spiropyran functionalized Au-NPs.

Enzyme Mediated Encapsulation of Gold Nanoparticles 15

E. Arbely graduated in Chemistry and received his Ph.D. in Biochemistry andBiophysics at the Hebrew University of Jerusalem in 2008. He was a post-doctoral fellow at the MRC Laboratory of Molecular Biology in Cambridge,where he worked under the guidance of Prof. Sir Alan Fersht and Dr. JasonW. Chin. Since 2012 he is a faculty member at the Department of Chemistry,Ben-Gurion University, Israel. His research activity is in the field of syntheticbiology, with particular interest in developing and applying methods forgenetic code expansion to study the structural and functional role of proteinpost-translational modifications.

I. Popov graduated in Physical Metallurgy and got her PhD in SolidState Physics at the Donetsk Physics and Engineering Institute in 1995. At1997–2001 she was a post-doc fellow at the group of professor Dan Shechtman(Nobel prize in Chemistry 2011) where she studied phase transformations andprecipitation of quasi-crystalline phases. From 2001 she is in charge of theUnit for Nanoscopic Characterization and the Hebrew University Center forNanoscience and Nanotechnology. She is an author of about 70 peer reviewedjournal papers. Her research activities are currently focused on developmentof advanced methods for characterization of materials.

16 R. Sfez et al.

I. (Shy) Arkin studied biology at the Hebrew University of Jerusalem andTel Aviv University and obtained his PhD degree from Yale University.His major research interests are membrane protein structure and function,alongside development of tools to investigate them. His group uses experi-mental approaches, as well as computational tools to analyze the mechanismof membrane transport systems that regulate salinity and acidity of humanpathogens. One example is using extensive computational simulations andexperimental assays his group elucidated the mechanism of the main Na+

pump of bacteria. Similar efforts in his group uncovered novel mechanismsfor drug resistance of the flu virus and potential ways to overcome it. Finally,his group is well known for developing isotope edited FTIR spectroscopy ingeneral, and the 13C=18O probe in particular for structural studies of proteins.

S. Yitzchaik graduated in Chemistry and got his Ph.D. in Chemistry atthe Weizmann Institute of Science, Rehovot in 1992. A faculty memberof the Hebrew University since 1996. He is an author of about 100 peerreviewed journal papers. Major research interests since 1996 focus aroundself-assembled nanolayers. He has authored over 100 papers and holds 20patents in the fields of material science, solid-state organic chemistry andnanolayers derived biosensor technologies.